Side topic: I assume you have 4GB of ram physically plugged into your motherboard. Have you tried to get use of all 4GB?
To use all 4GB, you need:
A motherboard chipset that supports more than 4GB physical address space. Most do, but a few don't.
A BIOS that supports more than 4GB of physical address space. Most do.
The correct BIOS settings, which aren't consistently named from one BIOS to another, that enable memory remapping.
A PAE kernel.
32 bit Windows limits you (by license restriction) to 3.2GB of ram even if you have all of the above details covered. So maybe you expected 32 bit Linux to be no better. But 32 bit Linux can support a full 4GB or even more of physical ram.
If you have a BIOS setting wrong or the wrong kernel for a full 4GB of ram, that is worth correcting. If it is a motherboard hardware restriction it can't be fixed without a new motherboard.
As for memory use (your actual question) see my answer at:
That gives just a bit more info than the usual answer such as pljvaldez posted above